In Plant is not possible to create circular railing, But you can use the AutoCAD commands to do this. Create one vertical pipe and use the polar array. After this sweep command to create top railing pipe and others...or use "donut" command to create the horizontal pipes.
The part of structure of Plant3D is don't have presision components, because is only use for have reference in 3D. If you want to create perfect structure you need to create it in AutoCAD Structural Detailing 2012.
Another option is to draw an AutoCAD polgon that has the same number of sides that the railings would have and then use the endpoints of the lines to place the sections of railings:
This is fairly quick and easy to do.